It is often necessary to provide an implanted pump to assist the heart of a human or a non-human animal subject with blood circulation, or as a substitute for the subject's heart. Examples of such implanted pumps include a ventricular assist device (VAD) implanted in a patient's body to take over some or all of the pumping function of the heart, as well as other cardiac assist devices and artificial heart pumps.
VADs having a brushless motor are known. In a known configuration, a VAD may include a housing and a stator having a plurality of motor windings disposed around the housing. The VAD also includes a rotor disposed within the housing. The rotor includes a magnet, typically a permanent magnet. The stator and rotor constitute a brushless motor. A rotating magnetic field is created and maintained by using an appropriate multi-phase sequence of excitations to supply the stator phases. The magnet in the rotor tends to align itself with the magnetic field, and thus the rotor spins about its axis and impels blood through the housing. Methods and controllers using variations of trapezoidal commutation for operating VADs are known. For instance, a 120° trapezoidal method using three phases involves exciting or driving each winding for 120° of the 360° electrical rotation cycle and leaving the winding un-driven for 60°.